This invention relates to magnet assemblies, and more particularly to an assembly in which the flux field of a magnet which normally attracts a ferromagnetic member can be shunted upon relative rotation of the magnet and a shunt member.
Permanent magnet assemblies which can be turned on and off by shifting elements relative to each other have been used in the past for chucks or work holders. Examples are found in U.S. Pat. No. 2,053,177 to Bower, issued Sept. 1, 1936, in which permanent magnets are shifted longitudinally between ON and OFF positions. In the ON position the poles of the magnet are aligned with pole pieces in the surface of the work holder. In the OFF position each pole piece bridges and thereby shunts a pair of magnetic poles. U.S. Pat. No. 2,187,240, issued Jan. 16, 1940 to Karasick, employs two rows of magnets and one row is shifted relative to the other to accomplish the turning ON and OFF of a chuck. U.S. Pat. No. 2,209,558 issued July 30, 1940 to Bing et al uses generally cylindrical permanent magnets which are rotated about their longitudinal axis within pole pieces to turn the chuck ON and OFF.
Another form of magnetic work holder is disclosed in U.S. Pat. No. 3,121,193, issued Feb. 11, 1964 to Engelsted et al. In such patent, alternate layers of ferromagnetic pole pieces and non-ferromagnetic spacers form a disc-like upper working surface. Beneath this upper working surface are alternate layers of permanent magnets and pole pieces. When the permanent magnets are aligned beneath the pole pieces of the upper layer, the work holder is ON. The work holder is turned OFF by angularly rotating the lower assembly of permanent magnets and pole pieces so that the upper pole pieces lie across several permanent magnets and thereby shunt the magnetic flux.
Each of the chucks or work holders use a large number of magnet poles and shunt pieces to exert a magnetic force over a relatively large surface area.
I have provided a permanent magnet assembly using a single pair of magnet poles which can be urged ON and OFF by relative angular rotation between the magnet and ferromagnetic members which act as pole extensions in one position and shunt the magnetic flux when in another relative angular position. The permanent magnet assembly is particularly useful as a door or cabinet latch.
Magnetic door and cabinet latches which are presently available simply mount a permanent magnet either on the door or the frame and mount a ferromagnetic plate where it can be contacted by the magnet when the door is closed. In some cases, the door or frame is itself formed of a ferromagnetic material so that a separate plate is not needed. While such existing latches are very simple in construction, they require for release the direct application of a manual force to overcome the magnetic attraction. As a result the latches are necessarily restricted in the attractive force which they can use.